1. Field
This disclosure relates to nanowire assembly methods. More specifically, the present disclosure describes methods for nanowire patterning and assembly methods to form circuits and other structures.
2. Description of Related Art
Nanowires are a class of materials in which the diameter or width of the nanowires itself varies from a few nanometers to a few tens of nanometers. Such dimensions may not be readily accessible using conventional lithographic patterning methods, but may be made accessible using nanowire patterning or nanowire materials growth methods. Typically, nanowires have a length that is at least 10 times larger than the width or diameter dimensions. In some cases, the nanowires length can be 1000 times (or more) larger than the width or diameter. While nanowires have a number of electronic applications that may not be shared by larger submicrometer or micrometer scale wires, the use of nanowires in electrical applications have several limitations.
First, it may not be easy to assemble nanowires into desired electrical circuit architectures. Most electrical circuit architectures can be optimized if the electronic components themselves can be organized with arbitrary complexity. However, practically all methods for arranging nanowires known in the art result in simple, periodic arrangements of the nanowires, such as arrays of nanowires aligned side-by-side.
Second, establishing electrical contact to nanowires may be difficult. A high quality electrical contact is described as an ohmic contact, meaning that electrical current through the contact varies linearly with the applied voltage (i.e., follows Ohm's law). Electrical contacts typically involve attaching metal electrodes onto the nanowires. In the general case, the metal electrodes and the nanowires are made out of different materials, and the nanowires may be semiconducting, while the metal electrodes are metallic. Also, in the general case, this materials mismatch between the nanowires and the contacting electrode material means that the two contacting electrode materials and the nanowires are characterized by different work functions. This leads to a barrier to current flow at the contact—this barrier is called a Schottky barrier. The barrier leads to a non-ohmic contact. There is no general approach to establish ohmic electric contacts to nanowires.
As indicated, establishing good electrical contacts between different materials is generally difficult, and is generally more difficult when one of the materials is a nanowire. If the nanowires are very heavily doped with impurities, then they can exhibit metallic behavior, and contacts to such nanowires will usually be ohmic. However, for nanowires that are lightly doped, as is required for field-effect transistors, chemical sensing, thermoelectric, and other applications, ohmic contacts to the nanowires are much more difficult to achieve. In addition, semiconductor nanowires can be either p-type or n-type conductors, and if a method is found to establish an ohmic contact to a p-type nanowires, for example, it does not follow that the same method will work for n-type nanowires.
Another application in which electrical contacts can play critical roles include the electrical contacts to superconducting materials. In many cases, proximity effects can dominate the observed response—this means that the superconducting properties of the nanowires can be dominated by the superconducting properties of the contacts, not of the nanowires themselves.
Methods for assembling or patterning nanowires to form periodic structures that are horizontally aligned with the surface of the supporting substrate are known in the art. The following literature provides representative examples of forming arrays of horizontally aligned nanowires:    1. S. W. Chung, G. Markovich, J. R. Heath, “Fabrication and Alignment of Wires in Two-Dimensions,” The Journal of Physical Chemistry B. 102, 6685 (1998).    2. Tao, A.; Kim, F.; Hess, C.; Goldberger, J.; He, R.; Sun, Y.; Xia, Y.; Yang, P.; Langmuir Blodgett Silver Nanowire Monolayers for Molecular Sensing Using Surface-Enhanced Raman Spectroscopy. NanoLetters 3, 1229 (2003).    3. Huang, J.; Kim, F.; Tao, A. R., Connor, S.; Yang, P. Spontaneous formation of nanoparticle strip patterns through dewetting. Nature Materials vol. 4, p. 896 (2005).    4. Whang, D.; Jin, S.; Wu, Y.; Lieber, C. M. Large-Scale Hierarchical Organization of Nanowire Arrays for Integrated Nanosystems. NanoLetters 3, 1255-1259 (2003).    5. Wang, D.; Chang, Y.-L.; Zhuang, L.; Dai, H. Oxidation Resistant Germanium Nanowires Bulk Synthesis, Long Chain Alkanethiol Functionalization, and Langmuir-Blodgett Assembly. Journal of the American Chemical Society, 127, 11871 (2005).    6. Michael Diehl, Rob Beckman, Sophia Yaliraki, and James R. Heath, “Self-Assembly of Deterministic Carbon Nanotube Wiring Networks,” Angew. Chem. Int. Ed. 41, 353 (2002).    7. Nicholas Melosh, Akram Boukai, Frederic Diana, Brian Geradot, Antonio Badolato, Pierre Petroff, and James R. Heath, “Ultrahigh density Nanowire Lattices and Circuits,” Science, 300, 112 (2003).
Nanowires may serve as etch masks so that the length and width dimensions of the nanowires, as long as the relative positions of the nanowires, may be translated to underlying materials to create nanowires in the underlying material. The height of the nanowires in the underlying material may be determined by the thickness of that material or by the depth that is achieved in the etching process. Examples of using nanowires as etch masks are provided is the following literature:    8. Choi, S H; Wang, K L; Leung, M S; Stupian, G W; and others. “Fabrication of nanometer size photoresist wire patterns with a silver nanocrystal shadowmask,” J. Vac. Sci. & Tech. A-Vac. Surf. And Films, 17, 1425 (1999).    9. S. H. Choi, A. Khitun, K. L. Wang, M. S. Leung, G. W. Stupian, N. Presser, S. W. Chung, J. R. Heath, A. Balandin, S. L. Cho and J. B. Ketterson, “Fabrication of bismuth nanowires with a silver nanocrystal shadowmask, J. Vac. Sci. Tech. A-Vac. Surf. And Films, 18, 1236-1328 (2000).    10. R Beckman, E. Johnston-Halperin, Y. Luo, N. Melosh, J. Green, and J. R. Heath, “Fabrication of Conducting Silicon Nanowire Arrays,” J. Appl. Phys. 96 (10), 5921-5923 (2004).    11. Jung, G.-H.; Johnston-Halperin, E.; Wu, W.; Yu, Z.; Wang, S.-Y.; Tong, W. M.; Li, Z.; Green, J. E.; Sheriff, B. A.; Boukai, A.; Bunimovich, Y.; Heath, J. R.; Williams, R. S. Circuit Fabrication at 17 nm Half-Pitch by Nanoimprint Lithography. NanoLetters, 6, 351 (2006).
As indicated above, the establishment of ohmic contacts may be needed for the desired performance of a circuit or system. The following literature discusses the establishment of ohmic contacts for nanowires:    12. Yang, C.; Zhong, Z.; Lieber, C. M. Encoding Electronic Properties by Synthesis of Axial Modulation Doped Silicon Nanowires. Science, 310, p. 1304 (2005).    13. Wu, Y.; Xiang, J.; Lu, W.; Lieber, C. M. Single-crystal metallic nanowires and metal/semiconductor nanowires heterostructures. Nature, 430. p. 61 (2004).    14. Weber, W. M.; Geelhar, L.; et al., Silicon-nanowire transistors with Intruded Nickel-Silicide Contacts. Nano Letters v. 6, p. 2660-2666 (2006).
Methods for the creation of nanopatterned films have been described in the art. Such methods include the utilization of closest packed arrays of spheres as masks for materials deposition. When a material is deposited onto the surface of an array of spheres, the region in between the spheres is also coated. The result is a patterned array of small particles. The size of those particles is dependent upon the way in which the material is deposited, and the size of the spheres that are used as masks. Another method for achieving nano-patterned substrates with extremely small feature sizes is to prepare an anodically etched alumina film. The resultant etched film may be characterized by a periodic array of holes that are controlled with respect to their size and their separation. These holes (or channels) may then be used as templates for depositing nanowire materials. Literature describing such methods includes the following papers:    15. “Nanosphere Lithography: A Versatile Nanofabrication Tool for Studies of Size-Dependent Nanoparticle Optics”, C. L. Haynes, R. P. Van Duyne, J. Phys. Chem. B, 105, 5599-5611(2001).    16. Routkevitch, D.; Bigioni, T.; Moskovits, M. Electrochemical Fabrication of CdS nanowires arrays in porous anodic aluminum oxide templates. The Journal of Physical Chemistry, v. 100, p. 14037-14047 (1996).    17. Martin, C. R. Nanomaterials—A membrane based synthetic approach. Science, v. 266, p. 1961-1966 (1994).
Methods for producing arrays of vertically aligned nanowires have been reported in the literature. Typically, such methods are limited to either semiconductor nanowires that are nucleated and grown from a metal nanoparticle seed, or carbon nanotubes that are similarly nucleated and grown. Literature that describes such methods includes:    18. Zhou, J.; Xudong, W.; Song, J.; Tummala, R.; Xu, N. S.; Wang, Z. L. Verticaly aligned Zn2SiO4 Nanotube/ZnO Nanowire Heterojunction Arrays. Small, v. 3, p. 622-626 (2007).    19. Peng, K.; Zhang, M.; Lu, A.; Wong, N-B.; Zhang, R.; Lee, S.-T. Ordered silicon nanowire arrays via nanosphere lithography and metal induced etching. Applied Physics Letters, v. 90, article # 163123 (2007).    20. She, J. C.; Deng, S. Z.; Xu, N. S.; Yao, R. H.; Chen, J. Fabrication of vertically aligned Si nanowires and their application in a gated field emission device. Applied Physics Letters, v. 88, article # 013112 (2006).    21. Fan, S.; Chapline, M.; Franklin, N.; Tombler, T.; Cassell, A.; Dai, H. Self-Oriented Regular Arrays of Carbon Nanotubes and their Field Emission Devices. Science, v. 283, p. 512 (1999).
One application of nanowire structures is the creation of suspended nanowires. Suspended nanowires may be used, for example, in high-frequency nanomechanical resonators or for thermal properties measurements. Arrays of suspended nanowires can have superior signal-to-noise performance over individual nanowires, but may be difficult to suspend because the nanowires themselves can exhibit strong nanowire-nanowire interactions that can cause the structure to collapse. In addition, for high frequency resonator applications, the frequency of the resonator depends upon the structural characteristics of the nanowires—for example, shorter nanowires are higher frequency resonators, if all other structural and materials aspects are kept the same. Literature that discusses suspended nanowires includes the following papers:    22. D. Y. Li, Y. Y. Wu, P. Kim, L. Shi, P. D. Yang, A. Majumdar, Appl. Phys. Lett. 2003, 83, 2934.    23. A. Husain, J. Hone, H. W. C. Postma, X. M. H. Huang, T. Drake, M. Barbic, A. Scherer, M. L. Roukes, Appl. Phys. Lett. 2003, 83, 1240.